Honeybees around the world are vanishing, tens of thousands at a time, and scientists don’t know why.

If the bees were dying of pesticide poisoning or freezing, their bodies would be expected to lie around the hive. And if they were absconding because of some threat — which they have been known to do — they wouldn’t leave without the queen.

Since about one-third of the U.S. diet depends on pollination and most of that is performed by honeybees, this constitutes a serious problem, according to Jeff Pettis of the U.S. Agricultural Research Service.

This is not a new problem. From 2004: “Bees, via pollination, are responsible for 15 to 30 percent of the food U.S. consumers eat. But in the last 50 years the domesticated honeybee population—which most farmers depend on for pollination—has declined by about 50 percent, scientists say.”